1. Field of the Invention
This invention relates to a metal gasket adapted to seal a narrow clearance between opposed surfaces of a cylinder head and a cylinder block and a narrow clearance between the cylinder head and an exhaust manifold, and comprising bead plates on which beads extending along combustion chamber bores and combustion gas bores are formed, and an intermediate plate interposed between the bead plates.
2. Description of the Prior Art
A conventional engine comprising a cylinder head and a cylinder block which are formed out of an aluminum alloy has small weight but a low rigidity. Therefore, relative displacement occurring between the cylinder head and cylinder block during an operation of the engine tends to become larger. In recent years, a metal gasket used for sealing a narrow clearance between a cylinder head and a cylinder block and formed by elastic metal plates is known on which beads are formed on the portions thereof which are in the vicinity of circumferences of holes therein made correspondingly to combustion chambers, water bores and oil bores.
In a conventional metal gasket disclosed in Japanese Patent Laid-Open No. 155374/1987, an intermediate plate is laminated on a compensation plate provided with combustion chamber holes, and edge portions of the combustion chamber holes in the compensation plate are folded, bead-carrying elastic metal plates being laminated on the compensation plate and intermediate plate, compensating portions of a predetermined thickness for preventing an increase and a decrease in the size of a clearance between deck surfaces being provided on the portions of the elastic metal plates which extend from the beads toward the combustion chamber holes.
As shown in FIG. 27, a metal gasket disclosed in Japanese Patent Laid-Open No. 293363/1988 is formed by laminating intermediate plates 45, 46 on each other between bead plates 41, 42 formed by elastic metal plates and having annular beads 43, 44 thereon which extend along the circumferential edges of combustion chamber holes 40 so that ridge portions of the beads are opposed to each other, and firmly fitting spacer members 47 between the portions of the intermediate plates 45, 46 which are closer to the combustion chamber holes 40 than the portions of the same which the beads 43, 44 contact. This metal gasket is provided with spacer members 47 fitted firmly between the regions of the intermediate plates 45, 46 which are closer to the combustion chamber holes 40 than the portions of the same which the beads 43, 44 contact.
As shown in FIG. 28, a metal gasket disclosed in Japanese Patent Laid-Open No. 65367/1989 is formed by laminating intermediate plates 55, 56 on each other between bead plates 51, 52 formed by elastic metal plates and having annular beads 53, 54 thereon which extend along the circumferential edges of combustion chamber holes 50 so that ridge portions of the beads are opposed to each other, and embracing a hole edge portion 57 of the intermediate plate 55 in that 58 of the intermediate plate 56 so as to form a grommet-like part of a stepped structure, whereby a folded portion 59 for offsetting the irregularity of a clearance between opposed surfaces is provided. In this metal gasket, substantially identical stepped portions are provided on the surfaces of the intermediate plates 55, 56, and the folded portion 59 works as a shim with respect to the beads 53, 54, when the bead plates 51, 52 are tightened onto the intermediate plates.
As shown in FIG. 29, a metal gasket disclosed in Japanese Utility Model Laid-Open No. 66457/1992 is formed by laminating intermediate plates 65, 66 on each other between bead plates 61, 62 formed by elastic metal plates and having annular beads 63, 64 thereon which extend along the circumferential edges of combustion chamber holes 60 so that ridge portions of the beads are opposed to each other, forming stepped portions 67, the height of which is less than the thickness of the intermediate plate 66, on the intermediate plate 65 so that the stepped portions extend along edge portions of the combustion chamber holes, engaging the portions of the intermediate plate 66 which is opposed to a non-stepped portion of the intermediate plate 65 with an outer surface of a recessed surface, which occurs due to the formation of the stepped portions 67, and forming compensation portions 69, the thickness of which is larger than the sum of the thicknesses of the portions of the two intermediate plates which the ridges of the beads contact, around the circumferences of the combustion chamber holes 60 by folding edge portions of the intermediate plate 66 between hole-edge portions of the intermediate plates 66, 65.
As described above, two intermediate plates are basically required in a conventional metal gasket to provide the gasket with compensation portions having the function of preventing a total compression of the beads on the portions of two bead plates which are at the circumferences of the combustion chamber holes, and the function of offsetting the irregularity of the opposed surfaces of the cylinder block and cylinder head which occurs when the gasket is tightened. A conventional metal gasket is provided with such compensation portions by using additional members, such as spacers or by folding parts of metal plates constituting the intermediate plates. This causes an increase in the number of parts and the number of steps, such as a welding step and a bonding step of a gasket manufacturing process, requires quality management concerning the precision of the compensation portions, and causes cracks and permanent set in fatigue to occur in the folded portions. This would result in an increase in the manufacturing cost.
A conventional metal gasket having one intermediate plate interposed between elastic metal plates is disclosed in, for example, Japanese Utility Model Laid-Open Nos. 4838/1989, 181756/1987 and 62668/1988. The intermediate plate disclosed in these publications has bulging portions or grooves for use in preventing the beads formed on the elastic metal plates from being totally compressed. However, such an intermediate plate is not formed to a uniform thickness due to the bulging portions of grooves, and, moreover, these bulging portions or grooves do not have the bead sealing function.